1. Field of the Invention
The present embodiments relate to offshore drilling and production platforms, particularly spar-type platforms.
2. Description of Related Art
Spar-type offshore drilling and production platforms typically include vertically elongated buoyant hulls. For example, FIG. 1 illustrates an example spar platform 100 having an outer hull 102 with a hollow centerwell 104 that is open to the sea at its lower end, and open to the atmosphere at its upper end. The hull 102 supports a deck (not shown) on which drilling and production equipment (not shown) may be mounted, along with other structures. The hull 102 includes a plurality of buoyancy tanks 106 surrounding the centerwell 104. The buoyancy tanks 106 define voids or compartments 108 that may be selectively filled with air or water to provide varying degrees of buoyancy to the platform 100. The buoyancy tanks 106 extend down to a truss structure 110, which, in turn, extends down to a ballasted keel 112. The ballasted keel 112 at the bottom of the truss structure 110 lowers the center of gravity of the platform 100 and improves the stability of the platform 100. One or more mooring lines (not shown) may be used to keep the platform 100 over its station.
Spar platforms are typically used in conjunction with one or more risers that extend under tension from the platform to a wellhead or an anchor on the seafloor. For example, the platform 100 of FIG. 1 includes top-tensioned risers (TTRs) 116. The TTRs 116 extend downward through the centerwell 104 from hydraulic-pneumatic tensioners (not shown) supported on a top-tensioned riser support frame 118. The hull 102 supports the support frame 118 above the surface 120 of a body of water (e.g., the sea). In alternative spar platforms, the TTRs may be supported by a buoyancy can (not shown) floating in the open centerwell. Such a configuration is disclosed in U.S. Pat. No. 6,176,646, the disclosure of which is incorporated herein by reference in its entirety. Alternative spar platforms may include catenary risers and/or bottom tensioned risers (BTRs) that are used to import oil and/or gas from remote fields or to export oil and/or gas to the shore or to other platforms. These risers are generally located in the open centerwell, and the platform may include pull tubes or containment tubes for surrounding and containing the risers. In some cases, catenary risers may be located on the outside of the platform and mm along the length of the platform. Other utility pipes that are open at the bottom may also be located in the centerwell.
As understood from FIG. 1, the centerwell 104 is open to the sea at its bottom and flooded with sea water. Accordingly, the centerwell 104 does not contribute to the buoyancy of the platform 100.